1. Field of the Invention
The present invention relates to the "direct" ignition of pulverized coal and particularly to the employment of an electric arc to initiate the combustion of a fuel stream comprised of pulverized coal entrained in primary air. More specifically, this invention is directed to the electrically produced ignition of a dense phase coal-air fuel stream with reliability and repeatability and without the use of any supplemental sources of ignition energy. Accordingly, general objects of the present invention are to provide novel and improved methods of such character.
2. Description of the Prior Art
Because of fuel cost and availability problems, it is becoming increasingly desirable to utilize coal rather than natural gas or oil in electricity generating facilities. Present day coal-fired steam generator boilers of the types employed by electrical utilities require, in order to insure safe and efficient operation, the use of premium liquid and gaseous hydrocarbon fuels to provide both ignition and low-load flame-stabilizing energy. By way of example only, it is not uncommon to consume 70,000 gallons of oil for one start-up of a 500 megawatt coal-fired generator unit. Obviously, the elimination of the need to consume such significant amounts of premium fuel in coal-fired plant is present and becoming increasingly urgent.
When compared to natural gas or oil, coal in ungasified form is a difficult fuel to ignite. In fact, until recently it was believed impossible to reliably directly ignite; i.e., to cause ignition in a cold furnace; a fuel stream comprising pulverized coal entrained in air. At this point in time the mechanism of ignition of pulverized coal is not fully understood. It is believed that ignition of coal particles is a function of their surface properties and/or result of the devolatization of gas from such particles. In any event, the initial ignitionary action is probably dependent upon heating rate and thus it is generally considered desirable to deliver high input energy to cause rapid heating with the subsequent release of a significant amount of volatile-forming molecules. In a furnace which has been preheated through the combustion of gas or oil, sufficient energy will be present to insure the ignition of all of the coal particles and the mechanism by which ignition of the individual particles occurs is of secondary importance. However, without a high energy source defined by an oil or gas fuel, which is in the ignited state when coal delivery is initiated, and the hot walls of the furnace, a coal-air fuel stream could not until recently be reliably ignited. Recent work, as exemplified by the disclosure of co-pending application Ser. No. 865,747 filed on Dec. 29, 1977 and entitled "Direct Ignition of Pulverized Coal", has demonstrated that under proper conditions the "direct"; i.e., without supplemental energy sources; ignition of pulverized coal may be reliably accomplished. The disclosure of such co-pending application Ser. No. 865,747, which is assigned to the assignee of the present invention, is incorporated herein by reference.
It is, of course, necessary that conditions be established in a coal-air fuel stream which, when sufficient ignition energy is delivered thereto, will cause a flame to propagate throughout the fuel mixture. Ignition of a cloud of pulverized coal is not a simultaneous event. A typical pulverized coal particle (-200+300 mesh) will burn out within one second. Thus, if a flame is to be propagated, the particles ignited by an ignition energy source must transfer energy to and ignite some of their neighboring particles. The criterion for successful ignition is the rate of heat generated in a flame pocket, which results from the delivery of energy to the fuel stream from the ignition source, must exceed the rate of heat loss due to endothermic devolatization and due to radiation and convection losses. The problem of insuring flame propagation becomes particularly acute where the ignition energy source, because of its nature, is operated intermittently. If freedom from the use of conventional gas and oil fuels is to be achieved, electrically powered sources of ignition energy are dictated. In the case of the most common type of electric energy source, which is a spark discharge, intermittent operation is virtually mandatory.
Spark ignition of combustible fuels has been the subject of considerable study. While it is known that the total spark energy which is available from an arc ignitor may be sufficient to ignite coal particles in the vicinity of the electrical discharge, the rapid discharge associated with conventional arc ignitors initiates a shock wave. Thus, the operation of a conventional spark rod in a fuel stream comprising coal particles entrained in primary air would appear to be undesirable since the shock wave generated at the time of the spark discharge would tend to push the coal particle away from the ignitor tip thus diminishing the possibilities of achieving ignition and subsequent propagation of flame.
As discussed above, in a "cold" furnace the fuel stream must be ignited as it is injected into the furnace. Thus, the problems of achieving "direct" ignition of a fuel stream comprising pulverized coal are aggravated by the fact that the fuel mixture is moving with a certain velocity. Additionally, there is apt to be turbulence in the fuel stream which, although it promotes flame propagation, also causes convective heat loss. This heat loss through convection, in fact, outweighs any advantages in flame propagation that may be derived because of the turbulence. The difficulties in achieving ignition of a moving fuel stream comprising pulverized coal can not be overcome merely by increasing the energy content of the spark utilized as the ignition energy source since the aforementioned problems associated with the creation of shock waves may be aggravated and the life expectancy of the ignitor is inversely related to spark current.